Add possible solution for StringsAndBinaries-4

I could've used parse.exs from the book, too, I guess?

strings/stringsandbinaries1.exs

@@ -12,4 +12,22 @@ defmodule StringsAndBinaries do
 	  # An anagram is a word or phrase formed by rearranging the letters of a different word or phrase, typically using all the original letters exactly once.
 	  Enum.sort(String.to_charlist(word1)) == Enum.sort(String.to_charlist(word2))
   end
+  
+  def calculate(expression) do
+	  # this might be a bit dirty, but it solves the issue.
+	  [a, operator, b] = ~w[ #{expression} ]
+	  a = String.to_integer(a)
+	  b = String.to_integer(b)
+	  case operator do
+		  "+" -> add(a, b)
+		  "-" -> substract(a, b)
+		  "*" -> multiply(a, b)
+		  "/" -> divide(a, b)
+	  end
+  end
+  
+  defp add(a, b), do: a + b
+  defp substract(a, b), do: a - b
+  defp multiply(a, b), do: a * b
+  defp divide(a, b), do: a / b
 end